1. Field of the Invention
The present invention relates to a photochromic composition which develops a neutral tint such as grey, green, brown or amber upon the irradiation with light including ultraviolet rays such as sunlight starting from the initial colorless state, maintaining a homogeneous color tone in the transient stage of developing color.
2. Prior Art
Photochromism is a phenomenon which is drawing attention in these several years, and stands for a reversible action of a compound; i.e., a compound quickly changes its color when it is irradiated with light containing ultraviolet rays such as sunlight or light of a mercury lamp, and resumes its initial color when it is no longer irradiated with light and is placed in a dark place. The compound having such a property is called photochromic compound. The compounds having a variety of structures have heretofore been synthesized and proposed, but no particular common skeleton is recognized in their structures. As the photochromic compounds, there have been known fulgide compounds or fulgimide compounds (hereinafter, fulgimide compounds and fulgide compounds are referred to as fulgimide compounds), chromene compounds and spirooxazine compounds.
The fulgimide compounds, at the start of developing color, exhibit color tones of, generally, orange to blue, the chromene compounds develop orange to yellow color, and the spirooxazine compounds develop reddish violet to blue color.
It is generally desired that the photochromic lenses develop color of a tone of grey, green, amber or brown. When the above-mentioned compounds are used in a single kind, however, a desired neutral tint is not obtained in many cases. Neutral tints can be obtained by mixing chromene compounds, fulgimide compounds and spirooxazine compounds that develop colors of different tones at any composition ratio. For example, Japanese Unexamined Patent Publication (Kokai) No. 121188/1991 discloses how to obtain neutral tints by mixing a chromene compound and a fulgimide compound, and Japanese Unexamined Patent Publication (Kokai) No. 9469/1993 discloses how to obtain neutral tints by mixing a chromene compound and a spirooxazine compound. Furthermore, German Patent Laid-Open Specification DE4325154 discloses how to develop grey, amber, brown as well as various other neutral tints by mixing a spirooxazine compound, a chromene compound and a fulgimide compound together.
In developing a color of a photochromic lens by placing it outdoors or by the irradiation with sunlight, it is one of the important properties of the photochromic lens that a color of a homogeneous tone is obtained in a transient stage in which the photochromic lens develops a neutral tint such as grey or brown starting from the colorless state.
A photochromic lens intended to develop a neutral tint by mixing several kinds of the above-mentioned photochromic compounds develops a color of which the tone changes from the colorless state to blue, to grey or brown when it is observed outdoors with the passage of time, and fails to develop a color of a homogeneous tone in the transient stage of developing color. The reason is because various photochromic compounds in the mixture composition exhibit different sensitivities for the sunlight. The fulgimide compounds or the spirooxazine compounds used in the above-mentioned known art exhibit good sensitivity for the sunlight compared to the chromene compounds. Therefore, they exhibit color starting from blue and finally exhibit a neutral tint of grey or brown. When it is attempted to obtain a neutral tint using photochromic compounds of different kinds such as chromene compounds and fulgimide compounds as described above, there arouses a problem in that the color tone lacks homogeneity in the transient stage of developing color.
When the above-mentioned chromene compound, fulgimide compound or spirooxazine compound is used alone, there is no problem concerning the homogeneity of color tone in the transient stage for developing color but a desired color tone is not obtained in many cases. When the compound is used as a photochromic lens, in particular, a color tone such as grey, green, amber or brown is preferred. Such a color tone, however, is not obtained by using the above-mentioned compound in a single kind.